Why Is My Car Misfiring After Wrong Fuel
Misfiring after wrong fuel occurs for various reasons, depending on whether the engine is misfiring due to the wrong fuel still being in the system, or if the drain was completed but the wrong fuel caused damage to components before it was removed. For each case, the cause is different and the solution is different.
At Wrong Fuel Help Australia we see both types of misfiring situation, vehicles that are misfiring because the wrong fuel is still active in the system and vehicles that continue to misfire after the drain has been completed. The reason for that is different in every case and the solution depends on what you’re dealing with. If your car is currently misfiring and has not been drained yet, that’s a different problem than a car that was drained yesterday and is still running roughly. This post is about both. If you’ve just topped up with the wrong fuel then start with our step-by-step guide which lays out exactly what to do from the moment you realise.
Misfiring With Wrong Fuel Still in the Tank: What Is Happening
A misfire is a combustion event that is not completed or fails in one or more cylinders. When the wrong fuel is present in the tank and circulating through the system, the wrong fuel is supplied to the cylinders, which leads directly to the failure of combustion. The engine management system detects that a combustion event did not result in the expected pressure increase in the cylinder and records a misfire fault code. The engine warning light can illuminate if there are several misfires in the same cylinder in a row.
Our guide to the full sequence of symptoms when wrong fuel is still in circulation through the system covers the full sequence of symptoms that appear when wrong fuel is in circulation. The misfire is one part of that sequence and where it falls in the progression helps you assess how urgent the situation is.
Why Petrol in a Diesel Engine Causes Misfiring
Diesel engines are compression ignition. It compresses the air-fuel mixture until the fuel ignites spontaneously from the heat of compression. Diesel has the auto ignition properties to do this reliably. Petrol is not so easily ignited by compression as is diesel. It takes a spark to get it started.
When petrol gets into a diesel injection system and makes its way to the cylinders, the compression ignition process becomes erratic. Some cylinders may not fire completely. Some may not fire at all. It is this inconsistency that causes the misfire sensation: rough running, hesitation under acceleration, a shaking or stumbling feeling from the engine, and in some cases complete stalling when the contamination proportion is high enough that no cylinder fires reliably.
On a modern common rail diesel the engine control unit is trying to make up for the inconsistent combustion by varying injection timing and quantity of fuel as well. This compensation can mask the misfire for a short time before things get worse as more petrol contaminated fuel is circulated in the system. If you’ve got petrol in a diesel engine in your vehicle, our dedicated service page explains the process of recovery and what happens when you get professional roadside fuel removal.
Why Diesel in a Petrol Engine Causes Misfiring
Petrol engines are spark ignited. The spark plug fires at the proper point in the compression cycle to ignite the air-fuel mixture. For this to work properly the fuel must vaporise easily to mix with the air and ignite from the spark. Petrol is designed to do that.
Diesel is a heavier, oilier fuel and will not vaporise well in the conditions inside a petrol engine cylinder. If diesel gets into a petrol system , the air-fuel mixture is too rich with dense , poorly vapourised fuel . The spark plug fires and the mixture is not burnt cleanly. This causes incomplete combustion, which the ECU registers as a misfire. The driver will notice a lumpy idle , hesitation and rough running . In some cases the engine is stalling and will only start cleanly once the diesel part in the tank drops below a limit.
Diesel in a petrol engine also contaminates the spark plugs. Diesel is more viscous than petrol and when it does not burn completely, it leaves deposits on the electrodes of the spark plugs. Even if the mixture is corrected, a diesel fouled spark plug may produce a weaker spark or misfire on its own and that’s where post-drain comes in. Diesel in Petrol Car recovery service Find out more If your car has been filled with the wrong fuel.
Misfiring After the Drain: Why It Can Persist Once the Fuel Is Out
This is the situation most drivers are not expecting and most online resources do not clearly address. The incorrect fuel was drained. Tank has been flushed. The right fuel has been put in. Engine starts but is still miss firing or running rough. Why?’
Contaminated fuel is removed from the tank and the accessible fuel system flushed out. It does not automatically reverse any changes made to components due to the circulation of the wrong fuel. Depending on the length of the operation and the type of fuel used, there may be residual effects in the system after the drain is completed.
Residual contamination in fuel lines and injectors
A fuel drain and flush removes fuel from the tank, lines and filter. The injectors themselves retain small quantities of fuel within their nozzle passages. If the engine has been running on contaminated fuel for some time, some of that contamination could still be in the injectors. This residue generally clears after a short period of running on clean fuel and a short period of rough running immediately after a drain and refuel is not unusual. If the misfire continues after the first few minutes of running on clean fuel then residual contamination in the injectors alone is unlikely to be the whole explanation. A proper professional drain typically clears residual contamination fast.
Fouled spark plugs from diesel contamination in a petrol engine
The most common cause of post-drain misfiring in petrol engines that have been dosed with diesel. Diesel deposits oily deposits on the electrodes of the spark plug during an incomplete combustion. These deposits degrade the plug’s ability to produce a clean spark and can cause a cylinder to misfire even after the fuel system is clean.
Fouled plugs may clear after long period of running on clean fuel at higher RPM which burns off deposits. They may also need replacement if the fouling is severe. If after a drain the misfire is clearly limited to certain cylinders (rather than just a general rough-running sensation) then you have a very strong indication that spark plug fouling is involved. A diagnostic scan with consistent misfire fault codes for the affected cylinders confirms the affected plugs.
Injector damage from petrol in a diesel system
If the diesel engine was run on petrol for any length of time the injectors may have been exposed to petrol without any adequate lubrication. Scored or worn injector nozzle seats will deliver fuel inconsistently which produces a misfire pattern that does not resolve with clean fuel and does not resolve with a second drain. This type of post-drain misfire exhibits itself with poor power and rough running under load and it typically persists regardless of fuel quality. A diagnostic scan showing live injection data and individual injector corrections will determine if one or more injectors are outside acceptable parameters. Understand when the wrong fuel can cause permanent damage to your engine or injectors and why some vehicles will need more repairs after the contaminated fuel has been drained.
When a Misfire After Wrong Fuel Means More Than Residual Contamination
A miss that goes away after running for a short while on clean fuel is consistent with residual contamination passing through the system. A misfire that persists beyond 10 to 15 minutes of running on clean fuel, particularly if it’s isolated to certain cylinders, or if there is a loss of power, is an indicator of component level damage that needs to be evaluated.
The diagnostic scan is the first step. Fault codes for specific cylinders, deviations in injection pressure or fuel trim values that are way off all point to components, not residual contamination. Driving through a persistent misfire after a drain will accelerate wear on any already-damaged parts, and it can cause secondary faults in the catalytic converter from unburned fuel passing through the exhaust.
In the event you have the wrong fuel and it causes damage that cannot be rectified by a drain, check out our guide to permanent misfuelling damage to see what level of assessment is required and when the scope of repair moves from a service item to a replacement.
Wrong Fuel Help Australia can help if your car is misfiring after a wrong fuel drain and the misfire isn’t fixing itself. We operate 24 hours a day, 7 days a week throughout Newcastle, Central Coast, Lake Macquarie, Maitland and the Hunter Region. Call Jay direct on 0432 553 905 or visit us at wrongfuelhelp.com.au. We can identify the cause of the persistent misfire and suggest appropriate next steps.
Frequently Asked Questions
How long should a misfire last after a wrong fuel drain?
A brief rough-running period can occur while residual fuel clears from the system. If the misfire continues after several minutes of normal operation or returns under load, the vehicle should be professionally inspected because component damage may be present.
Can I clear the misfire fault codes myself with a code reader?
You can clear the codes, but doing so doesn’t fix the cause. If the underlying fault remains, the warning light and misfire will return. Reading the stored codes before clearing them provides valuable diagnostic information.
Will the misfire damage my catalytic converter?
A prolonged misfire can overheat the catalytic converter because unburned fuel enters the exhaust system. Continuing to drive with a persistent misfire increases the risk of additional damage.
Is the misfire always the engine’s fault or could the fuel drained correctly?
Both are possible. Residual contamination usually clears quickly after the drain. A persistent misfire often indicates fouled spark plugs, injector damage or another fuel system issue that requires diagnosis.
Why did the misfire appear after the drain rather than during the wrong fuel incident?
Diesel deposits or injector wear may not become obvious until the engine is running on clean fuel again. This is why some vehicles begin misfiring after the drain rather than during the original incident.
